PATENT NUMBER: 6,137,161 ASSIGNEE: IBM INVENTORS: Don A. Guilliland and James L. Peacock TITLE: Interposer Array Module for Capacitive Decoupling and Filtering By now, it has become painfully obvious to most chip designers (and others at the front end of IC production) that the continuing increase in IC speed and performance strongly impacts the design of the package. This patent describes the use of an interposer package that employs an enhanced substrate that functions both as a power supply decoupling capacitor and, at the same time, provides individual capacitors to filter and condition each signal I/O. The interposer and IC assembly are then enclosed in a four-sided metallic cap that functions as an EMI shield. Interposer Circuit To simplify illustrating the various functions of this interposer, different colors are used in the accompanying figure. The dark reddish-brown color indicates the ground via and ground planes used throughout the substrate. Note that the interposer makes extensive use of ground planes. The blue via and plates indicate one power connection. Because of the need to keep power connections short, the decoupling capacitance is located as close to the surface of the IC as possible. And since larger capacitance values are required, these capacitor plates tend to extend largely across the entire interposer except around signal and ground vias. The close location of the capacitor plates to the IC also minimizes inductance.
The lighter red vias and plates represent signal circuits and filter capacitors. The values of capacitance on the signal lines are much smaller than those used for power supply decoupling, and they can be located farther away from the chip. The combination of a via and the related capacitor plates forms a low-pass feed-through filter. This filter is designed to establish the desired frequency roll-off while passing only the desired signal. The unwanted frequencies are shunted to ground. The capacitor plates and the ground planes are provided with sufficient clearance around the signal vias to prevent the plate potential or ground from having an uncontrolled influence on the signals carried by the signal lines. Two colors are used to indicate the interposer dielectric. Because different capacitance values are required for power decoupling, compared to signal filtering, it may be desirable to use different types of dielectrics to achieve optimum capacitance values for each function. Conductive Cap and EMI Shield The assembled circuit is enclosed by a continuous metal shield that extends across the top and along the four sides of the chip and interposer. It is mechanically attached to the assembly by a flange on the bottom and by a series of tabs that clamp to the top of the substrate. The flanges and tabs meet ground connections on the interposer to provide a ground potential to the EMI shielding.
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